Typical known coatings for automobile mirrors usually consist of relatively highly reflecting metals, such as, for example, silver or aluminium. The reflection values achievable with them for automobile mirrors amount to more than 85% in the visible light spectrum. Although, however, relatively high reflection values are desirable in the daylight, they might glare the driver at night through the headlights of the following vehicles. Therefore, mirror coatings were developed which are designed to reduce the glaring effect when driving at night. For this purpose, one can use, on the one hand, mirror systems which have a relatively low glaring effect at night, due to suitably designed reflecting elements, using in particular spectrally selective reflection coatings. On the other hand, one can also use rearview mirrors in which the risk of glaring through the headlights of the following vehicles is reduced by the fact that the mirror structure contains parts of variable transmission which are switched, for example, electrically.
In rearview mirrors designed in such a way, a reflection part with variable reflectance is used, using, for example, electrochromic elements or LCD elements. These can be integrated in the layer structure of the mirror system, for example, in the manner of an intermediate layer, it being possible to vary the transmission of the respective optically active layer by supplying it with a suitable chosen control signal and to modify in this way the reflectance of the reflection part of the rearview mirror.
In such systems, usually a multitude of design targets have to be taken into account. On the one hand, it is desirable to keep the glaring of the driver through the headlights of the following vehicles as low as possible. For this purpose, the rearview mirror should be switched to relatively dark, i.e. to a relatively low reflectance, when driving at night. On the other hand, however, also and especially at night, it should be guaranteed that the environment, i.e., for example, the surroundings of the vehicle, is perceived as reliably as possible, for which purpose a relatively high reflectance of the mirror is required. In order to provide the optimum compromise between these design targets, control or check units can be used in rearview-mirror systems with a rearview mirror provided with a reflection part with variable reflectance, which supply the reflection part with a control value suitably chosen under certain conditions.
In order to suitably take into account the ambient-light and glaring conditions, such rearview mirrors are usually provided with at least two light sensors, the first one measuring the illuminance impinging on the rearview mirror from the forward direction, which is representative for the background or ambient light, and the second one measuring the illuminance impinging from the backward direction, which is characteristic of the glaring through the following vehicles. The measured values supplied by these sensors can be used for specifying a suitable nominal value for the reflectance of the rearview mirror, the effects of usually quickly changing lighting conditions on the one hand and the inertia of the human eye due to physiological reasons on the other hand being taken into account or compensated through suitable averaging of the obtained measured values and in particular through suitably chosen low-pass circuits and the like.
A rearview-mirror system of this type is known, for example, from U.S. Pat. No. 4,917,477, which is incorporated by reference herein. This system uses, in addition to suitably chosen low-pass filters, static performance characteristics, in order to provide through suitable linking of the determined illuminance of the ambient light and of the glaring light a particularly favorable nominal value for the reflectance of the rearview mirror.